Is it possible to have more than one smoke simulation in the same scene?
It is possible.

You can create multiple working domains, but the whole process seems a bit awkward and only sometimes works, without any recognizable pattern.

One way I found that seems to consistently work as expected is to create your domains on separate layers.
If you need more than 20 domains, they sometimes work on the same layer, but I have not found what it is that makes/breaks them when they are on the same layer.
Most of the time it seems to work if the one domain that runs in real time is baked first, then the next domain that runs in real time, and the next, etc.


Create domains, on separate layers. Note that Separate domains can share the same Flow objects; however, they will not interact. 





Here, each domain is on a separate layer, and each flow object is visible on all layers with a domain object.
Bake each one individually:

It seems that the selected domain is always displayed above the others. Here is the domain that covers all the inflow objects selected (top) and unselected (bottom) so it's more visible that the domains share the same inflow objects: